(1) Field of the Invention
The present invention relates to a high strength wet-laid nonwoven fabric and a process for producing same. More particularly, the present invention relates to a nonwoven fabric having a high strength, e.g., a high tensile strength, a high tear strength, and a high interlayer peeling strength, and produced by a paper-making method, and a process for producing same.
(2) Description of Related Art
Nonwoven fabrics are now used in various applications and have replaced conventional knitted fabrics and woven fabrics or the like, since nonwoven fabrics have functional applications that can not be achieved by the conventional knitted fabrics and woven fabrics or the like, and the application of the nonwoven fabric has remarkably increased.
Various types of the nonwoven fabrics are known, for example, the following are typical known nonwoven fabrics; dry-laid nonwoven fabrics composed of filaments and obtained by a direct spinning of a fiber formable high polymer by a spunbond process, a flash spinning process or the like, drawing simultaneously spun filaments in the presence of a gas, such as air, and accumulating the obtained filaments. Such nonwoven fabrics are disclosed in Japanese Examined Patent Publication (Kokoku) No. 48-38025 and No. 42-19520, and Japanese Unexamined Patent Publication (Kokai) No. 63-50512. A dry-laid nonwoven fabric composed of staple fibers having a relatively long fiber length and obtained by a melt blowing process is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 49-48921 and U.S. Pat. No. 3,379,811. Dry-laid nonwoven fabrics composed of staple fibers and obtained by opening the staple fibers by a carding process, accumulating the opened staple fibers in a sheet form by using a cross laying machine or an air laying machine, and bonding the staple fibers constituting the sheet to each other by a needle punching process, an entangling process performed by columnar water streams, or an adhering process using an adhesive or heat feasible fibers are disclosed in Japanese Examined Patent Publication (Kokoku) No. 57-58463, U.S. Pat. Nos. 3,403,862 and 3,493,462. Further a nonwoven fabric produced by a paper-making method is well known.
Since fibers constituting the nonwoven fabric in the filament dry-laid nonwoven fabric are filaments, a nonwoven fabric obtained by a heat-press bonding of a web of filaments has a high tensile strength and tear strength, and thus this nonwoven fabric can be widely used as an industrial material for which a high strength is required.
Nevertheless, the interlayer peeling strength of this nonwoven fabric, i.e., the strength required to peel one layer constituting the nonwoven fabric from an adjacent layer is 300 g/cm to 400 g/cm at most and is not sufficient for a product made of the nonwoven fabric. It appears that this inferior interlayer peeling strength prevails because the fibers constituting the nonwoven fabric are filaments, and because the bonding between the filaments is two dimensionally applied by only the heat-press bonding process, and thus there is little entanglement between the filaments. Further, this nonwoven fabric has the following disadvantages. Namely, since a sheet of this nonwoven fabric is formed by applying a drawing and accumulating process using an air stream or a gas stream, the sheet does not have a required uniformity, or a weight per unit area of the sheet is very irregular, and since the bonding between the filaments is obtained by heat-press bonding, the resultant nonwoven fabric has a low elongation, a hard handling, and inferior drape characteristics.
The staple fiber dry-laid nonwoven fabric produced by using a card has less strength than that of the filament dry-laid nonwoven fabric, due to a short length of the fiber used, and when an adhesive or the like is used to provide a stronger bonding of the fibers constituting the nonwoven fabric, and thus increase the strength, a disadvantage arises in that the handling of the nonwoven fabric becomes very hard.
A "spunlaced" nonwoven fabric, i.e., a nonwoven fabric obtained by entangling fibers in a sheet formed by a card, by a water jet without an adhesive, has a soft handling superior to that of the spun bond type nonwoven fabric and a nonwoven fabric obtained by bonding fibers in the sheet formed by the card by using an adhesive or heat fusible fibers. Nevertheless, this nonwoven fabric has disadvantages such that the uniformity of the nonwoven fabric or irregularity of a weight per unit area of the nonwoven fabric is unpreferable due to the use of the card type sheet forming process, in that the interlayer peeling strength of the nonwoven fabric is still too low.
Since a sheet from which the wet-laid nonwoven fabric is made is formed by dispersing fibers having an extremely short length in water, this nonwoven fabric has a remarkable uniformity for superior to that of the dry-laid nonwoven fabric, but since in general, fibers having an extremely short length, e.g., 3 mm to 7 mm, must be used to ensure a uniform dispersion of the fibers in the water, the strength of the nonwoven fabric obtained by this method is very low, and therefore, the application of this nonwoven fabric is limited to fields in which a nonwoven fabric having a high strength is not required. Further, when a paper making machine is used for implementing this method, since the sheet is generally pressed by a dryer equipped with a felt or a yankee machine, a thickness of the wet-laid nonwoven fabric is thin and a density of the fibers in the nonwoven fabric becomes high, and thus the nonwoven fabric has a paper-like handling feel. These properties are typical disadvantages of the known wet-laid nonwoven fabric.
As described above, the conventional nonwoven fabric has advantages and disadvantages, depending upon the producing method used, and a nonwoven fabric having a superior uniformity, a high strength, and a soft handling has not ben produced to date.